1. Field of the Invention
The present invention relates to a current detection circuit which uses a current mirror circuit to indirectly detect a current.
2. Description of the Related Art
Current detection circuits are used in various applications such as motor drivers, power supplies, and the like. In a current detection circuit, in general, a resistor is inserted in a current supply path to a load, and current is directly detected from the two end-voltages thereof, but a method of indirect detection using a current mirror circuit is also being attempted. FIG. 1 of Patent Document 1 discloses a current mirror circuit 1 formed of transistors 3 and 2. When a size ratio of the transistors 3 and 2 is n:1, current flowing in the transistors 3 and 2 also has a ratio of n:1. When a predetermined voltage is given to a control terminal 4, the transistor 3 supplies a current IL1 to a load 13, and the transistor 2 supplies a detection current IS1=IL1+n to a resistor 11 via a transistor 9. Consequently, if the resistance value of the resistor 11 is R, a detection voltage R×IS1=R×IL1÷n is outputted at a detection terminal 12.
As described in Patent Document 2, this type of circuit can detect a load current with good accuracy and with no loss, without interposing a sense resistor in a load current path, and when detecting the load current, it is possible to easily switch gain in accordance with the size of the load current.
Patent Document 1: Japanese Patent Application, Laid-Open No. 2001-264365
Patent Document 2: Japanese Patent Application, Laid Open No. H7-113826
In FIG. 1 of the abovementioned Patent Document 1, with the transistors 3 and 2, which make up the current mirror circuit 1, in an unsaturated state, a current IDS between source-drain terminals has a dependency with a voltage VDS across the source-drain terminals. Consequently, a feedback circuit 5 formed of an operational amplifier 6 and a transistor 7 performs control so that voltages VDS across the source-drain terminals of the transistors 3 and 2 become equal.
However, since capacity for phase compensation is necessary inside the operational amplifier 6 included in the feedback circuit 5, when this type of feedback circuit is provided, the circuit size becomes large.